Methods and apparatuses for analyzing formations surrounding boreholes and processing resulting well logging data have been extensively described in the art. Note for example the U.S. Pat. No. 3,457,544 to Miller et al. In this patent a system is described wherein a generally cylindrical borehole investigating device provides well logging data from longitudinally distributed sensors which measure various surrounding borehole soil characteristics. The sensors may be electrode arrays and coil arrays for measuring electrical resistivities or conductivities of the subsurface formations, various radioactivity devices for measuring different nuclear phenomena in a borehole, or acoustic measuring devices or any combination of these or other borehole measuring devices.
The longitudinal spacing of the sensors in the borehole investigating device results in a depth-offset (and thus also a time-offset) between the well logging data signals with respect to any one particular borehole depth level.
As described in the above referred to Miller et al patent, the sensor signals are referenced to the depth of the sensors in the borehole with the use of a cable movement sensor. This latter sensor is associated with the cable from which the sonde is suspended in the borehole and generally produces well depth signals representative of cable movement of sufficiently small increments to satisfy depth resolution requirements of the well logging data signals.
Since the longitudinal spacings of the sensors on the borehole investigating device results in a depth-offset between the well logging signals with respect to any one particular borehole region, various techniques have been proposed to restore depth correspondence between the well logging data signals. In one known technique such as described in U.S. Pat. No. 3,714,623 to Mickler, a capacitor variable delay network is used to depth shift analog well logging data signals. Other U.S. patents describing devices and systems for depth matching well logging data signals are U.S. Pat. Nos. 3,405,349 to Moran and 3,434,105 to Schwartz.
Schwartz discloses a system wherein a digital data word is generated representative of an acoustic borehole characteristic over an incremental depth. The digital data words of successive increments are passed in sequence along parallel shift registers to enable borehole analysis by selecting digital data words for any desired borehole segment length. The digital data words are sequenced along their shift registers in synchronization with related intervalometer or well depth signals.
In another known approach for depth shifting of well logging signals, the latter are recorded on a magnetic medium such as a magnetic drum that is moved in synchronization with the movement of the sonde. The recorded signals are then read-out with magnetic heads whose spacings correspond with the spacings of the sonde sensors. The signals from the reading heads at any one instant in time are then representative of the same depth.
Computer processing to provide well depth alignment of well logging data is known. The use of shift registers to advance data at different speeds, or multiplexing of shift lines of shift registers has been done in the art in, for example, the field of telemetering of data.